As well known, loud speaker systems are characterized by acoustic-electric transducers, e.g., microphones, which respond to an audio, acoustic signal to derive an electric signal that is supplied to an amplifier. The amplifier drives an electric-acoustic transducer, e.g., a loud speaker, which generates an acoustic signal that is an appropriate replica in frequency, phase and amplitude of the acoustic signal supplied to the acoustic-electric transducer. The electric-acoustic transducer is frequently positioned relative to the acoustic-electric transducer such that positive acoustic feedback occurs between them. The resulting positive feedback is quite objectionable and manifests itself as "howling", a phenomenon that has been frequently encountered in many loud speaker system situations.
Many different devices and techniques have been employed for suppressing acoustic positive feedback to prevent howling. Amongst these methods and techniques are noise cancelling and directional microphones, baffles, and acoustic delay lines. Each of these prior art structures and techniques has certain problems associated with it.
Noise cancelling and directional microphones are designed to limit the positive acoustical feedback energy which can be coupled from the electric-acoustic transducer to the acoustic-electric transducer. It has been found that these structures do not always provide adequate suppression of frequency and phase components which are coupled between the transducers. This structure provides only a limited margin from the positive acoustic feedback and therefore does not perform adequately in certain situations.
The baffles and sound absorbing barriers are placed in a region where the loud speaker system is located in such a manner as to attenuate residual acoustic energy which would otherwise ultimately be coupled back in phase to the acoustic-electric transducer to produce oscillation, resulting in howling. In many instances, baffles can not be used, have limited effect, and are cumbersome. In a portable loud speaker system, baffles are usually out of the question because the region where the loud speaker system is operating may be outdoors, or in an enclosed structure that is susceptible to many other uses. A further disadvantage of the baffles is that they are bulky and require spatial or physical positioning.
Acoustic delay lines employed to prevent howling delay an acoustic signal supplied to a microphone prior to the signal being transduced into an electric signal. The acoustic delay line thereby time delays the originally generated acoustic signal and the electric signal supplied by the acoustic-electric transducer to the loud speaker system amplifier. Thereby, the acoustic signal derived from the electric-acoustic acoustic transducer is phase displaced relative to the originally derived acoustic signal. It has been found that this apparatus and technique is not entirely successful in many cases because feedback between the two transducers translates the problem, causing the acoustic signal to still be fedback in phase to the acoustic-electric transducer. To enable the acoustic delay line to be completely effective requires time delays which may produce echo effects unacceptable to a speaker and/or listener. The echo effect occurs because the acoustic signal must be time delayed for an interval greater than the time required for acoustic energy components to be reflected from the loud speaker to the microphone.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for preventing acoustic feedback i.e., howling, in loud speaker systems including acoustic-electric and electric-acoustic transducers.
Another object of the present invention is to provide a new and improved apparatus for preventing acoustic feedback in loud speaker systems wherein adequate suppression of acoustic feedback is provided with a relatively high margin from positive acoustic feedback.
Still another object of the invention is to provide an echo free apparatus for preventing acoustic feedback in loud speaker systems.
Still another object of the invention is to provide in-line electronic circuitry for preventing acoustic feedback in loud speaker systems.
An additional object of the invention is to provide a new and improved portable apparatus for preventing acoustic feedback in a loud speaker system, wherein the system is adaptable to any physical site, is not bulky and can be easily incorporated as a circuit component in connection with the loud speaker amplifier.